A jet engine produces a fast moving exhaust to produce thrust. Typically, a compressor pulls ambient air into a combustion chamber where it is mixed with a fuel to generate a hot exhaust gas. This exhaust gas then exits the engine and by Newton's law of equal and opposite reaction produces a forward thrust. A turbine element in the path of the exhaust then is spun by the exhaust gas. The turbine is connected by a shaft to the compressor, thereby allowing more air to be drawn into the engine in a Brayton cycle combustion process. Engine thrust depends upon exhaust gas velocity and mass flow rate of gas.
Jet engines contain afterburners which give the jet additional thrust. In some operating modes, such as take-off, supersonic flight, and combat situations, an increase in thrust is needed. This is particularly important in a combat situation when the pilot may need to quickly maneuver away from enemy fire. For these instances the afterburner is used. Thrust in an afterburning engine is increased by increasing both the mass of exhaust gases and the exhaust velocity.
As recognized by those skilled in the art of jet and rocket engine design, the use of afterburners requires a tremendous amount of additional fuel. Fuel is injected into the jet exhaust downstream of the turbine. This can increase thrust, but is very fuel inefficient, as the afterburner does not bum the fuel as efficiently as does the combustion section of a jet engine. A typical afterburner operates with exhaust gas that has limited oxygen content, which is insufficient for the complete combustion of the injected fuel, so some of the fuel is left unburned as it exits the afterburner.
A jet engine operating without the use of an afterburner is referred to as operating in dry mode, whereas an engine with an afterburner operating is referred to as operating in wet mode. When an afterburner is activated, large amounts of fuel are injected into the high-temperature, oxygen-deplete exhaust gases from the upstream combustion chamber(s) and turbine. Since the air has been used for combustion in the upstream combustion chamber(s), there is an insufficient amount of oxygen remaining to completely combust the large amount of fuel introduced in the afterburner. The use of afterburners, therefore, is a very inefficient process that wastes large amounts of fuel and introduces the volatile organic compounds into the atmosphere, usually at high altitudes. It is therefore desirable to have a device or means by which this added thrust can be achieved without having to use such tremendous amounts of fuel.
It is also known to those of ordinary skill in the art that heat-seeking missiles hone in on the hot areas of a target including the tailpipe and/or afterburner, exhaust nozzle, and the exhaust gas plume. The exhaust gases leaving the tailpipe form a plume that expands and cools. The jet exhaust may have a temperature between approximately 650° C. and 1300° C. depending on the type of engine and its operating conditions. On dry thrust, the tailpipe is the strongest radiator, and the plume is cooler, typically around 650° C. Using the afterburner may produce an exhaust plume of around 2000° C. or more.
Due to the high exhaust velocities needed for efficient jet engines, these engines generate a large amount of noise as their exhaust gases interact with ambient air creating shockwaves. A system is needed to minimize these developing shockwaves to reduce noise. Combustion gases exiting a jet engine also produce a great deal of heat which can be tracked by heat seeking missiles in military applications. Therefore a means to cool the exiting exhaust without limiting engine performance is also needed.